Dissociation Energies, Vibrational Frequencies, and ¹³C NMR Chemical Shifts of the 18-Electron Species [M(CO)₆]n (M = Hf−Ir, Mo, Tc, Ru, Cr, Mn, Fe). A Density Functional Study

Density functional theory has been used to calculate dissociation energies, vibrational frequencies, and 13C NMR chemical shifts of the following isoelectronic metal hexacarbonyls: [Hf(CO)6]2-, [Ta(CO)6]-, W(CO)6, [Re(CO)6]+, [Os(CO)6]2+, [Ir(CO)6]3+; Mo(CO)6, [Tc(CO)6]+, [Ru(CO)6]2+; and Cr(CO)6, [Mn(CO)6]+, [Fe(CO)6]2+. The first CO ligand dissociation energy ΔH follows the ordering Ir > Re ∼ Os > Hf ∼ Ta ∼ W through the third transition series. A decomposition of ΔH into contributions from the CO to metal σ-donation and metal to CO π-back-donation reveals that this trend is the result of a stronger σ-donation in the more oxidized systems. An increase in ΔH toward higher oxidation state is also apparent for the limited sample of 3d and 4d systems. Within a triad, the 4d metal forms the weakest M−CO bond. The calculated CO stretching frequencies are in good agreement with experiment. Further, CO stretching frequencies, optimized R(C−O) distances, and the calculated contribution to ΔH from the π-back-donation all reveal the expected decline in π-back-donation toward the more positively charged systems. Both experimental and calculated 13C NMR chemical shifts diminish with increasing oxidation state. It was shown that the stretch of CO on coordination and π-back-donation have positive (paramagnetic) contributions to the chemical shift, δ, whereas σ-donation has a negative (paramagnetic) contribution to δ. All factors contribute to the decline in δ with increasing oxidation state, although π-back-donation is predominant.